The present invention relates to a method and an apparatus for producing a saponified ethylene-vinyl acetate copolymer (hereinafter abbreviated as xe2x80x9cEVOHxe2x80x9d).
In production of an EVOH by saponification of an ethylene-vinyl acetate copolymer (hereinafter abbreviated as xe2x80x9cEVAcxe2x80x9d), it has been an object for a person skilled in the art to produce efficiently an EVOH having a high saponification degree in a short time. For this object, various methods have been proposed. Specifically, known methods include a method of saponifying EVAc in a methanol solution in the presence of an alkali catalyst in a tower reactor (Japanese Published Examined Patent Application No. Sho 43(1968)-14958); a method of saponifying an EVAc in a methanol solution in the presence of an alkali catalyst and further saponifying (re-saponifying) the EVAc in an aqueous solution of at least 50xc2x0 C. in the presence of an alkali catalyst (Japanese Published Examined Patent Application No. Sho 45(1970)-34152); and a method using a mixture of a water-soluble lower alcohol and water as a solvent for re-saponification (Japanese Published Unexamined Patent Application/JP-A No. Sho 48(1973)-43493. In the methods including re-saponification, a partially-saponified product separated from a reaction solution by means of centrifugation or the like is introduced into a separate solvent in which saponification is carried out again.
An alkali catalyst used for saponification of EVAc may cause degradation of the polymer such as caustic-soda corrosion or coloring. An acetic acid compound as a by-product remaining in the product will reduce the thermal stability, and cause coloring and generation of gels and hard spots. It is therefore desirable to decrease the amount of the alkali catalyst used. However, since simply decreasing the amount of the alkali catalyst used will lower the rate of the saponification in the above-described conventional methods, this decrease in the amount used should be limited.
Since the solubility of an EVAc having a low ethylene content in a saponification solvent such as methanol decreases as the saponification proceeds, a reaction under conditions of high temperature and high pressure temperature and high pressure apparently increases the above-described by-product, coloring and generation of hard spots.
After keen studies in view of the above issues, the inventors found that an EVOH having a high saponification degree can be produced efficiently with an alkali catalyst used in a smaller amount when compared to conventional methods, by selecting reaction conditions suitable to respective stages during saponification corresponding to differences in reaction mechanisms between initial stages and final stages of EVAc saponification, and to variations in solubility of the EVOH with respect to the solvent depending on the progress of saponification. While this method is applicable regardless of the ethylene content, it is particularly effective when applied to an EVOH having a small ethylene content.
Specifically, the present invention provides a method for producing an EVOH by saponification in a methanol solution containing an alkali catalyst, the method including supplying a solution containing a partially-saponified product of EVAc, obtained by saponification in a first reactor under a predetermined pressure, from the first reactor to a second reactor, and saponifying further the partially-saponified product in the second reactor under a pressure higher than the predetermined pressure in the first reactor so as to provide a final saponified EVAc product.
According to the method of the present invention, consumption of an alkali catalyst is suppressed in an initial stage of saponification of EVAc, and the saponification can proceed even in a final stage of the saponification. This method does not require an operation to separate a partially-saponified product from a reaction solution and introducing this partially-saponified product into a separate solvent. Therefore, an EVOH having a high saponification degree can be produced efficiently with a small amount of alkali catalyst.
It is preferable in the method that the ethylene content of the EVAc is from 10 mol % to 60 mol %, and more preferably, from 10 mol % to 30 mol %. As mentioned above, saponification of an EVAc having a lower ethylene content according to a conventional method will cause problems of by-products such as acetic acid compound, coloring and generation of hard spots. Since this method requires less alkali catalyst, these problems can be avoided easily.
In the method of the present invention, preferably a pressure in the first reactor is from 0.1 MPa to 0.2 MPa, and a pressure in the second reactor is from 0.2 MPa to 2.0 MPa. A preferable saponification degree of the partially-saponified product to be supplied to the second reactor is from 60 mol % to 98 mol %. It is preferable that the saponification degree of the final saponified product is raised to 99.0 mol % or higher.
In the method of the present invention, an alkali catalyst can be used in a range from 0.01 to 0.03 in molar ratio to an acetyl group in the EVAc. If the amount of the alkali catalyst is excessive, visible imperfections such as coloring, generation of gels and hard spots and fish-eyes might occur.
The alkali catalyst can be supplied to the first reactor alone, or it can be supplied to both the first reactor and the second reactor. Supplying to only the first reactor is advantageous in ease of controlling the total amount of the alkali catalyst used. Supplying to both the first and second reactors is advantageous in reducing the total amount of the alkali catalyst used.
When supplying the alkali catalyst to the first reactor alone, it is preferable that a solution supplied from the first reactor to the second reactor contains the alkali catalyst in a range from 0.005 to 0.03 in molar ratio to an acetyl group contained in the partially-saponified product. An excessive amount of alkali catalyst may lead to the above-described imperfections. On the contrary, when the amount is insufficient, saponification in the second reactor may not proceed sufficiently.
When the alkali catalyst is supplied to the second reactor as well, the second reactor can be a tower reactor that is divided into at least two regions in a direction from top to bottom thereof, and the alkali catalyst is supplied to the respective regions. Accordingly, the total amount of the alkali catalyst used can be reduced by separately supplying the catalyst to the second reactor.
It is preferable in the method of the present invention that the first reactor and the second reactor are tower reactors. By using tower reactors, saponification of the EVAc can be carried out continuously and efficiently. In this case, a methanol solution of an EVAc and a methanol solution of an alkali catalyst can be supplied through an upper portion of the first reactor, and then, a methanol solution containing a partially-saponified product of the EVAc and the alkali catalyst can be supplied from a lower portion of the first reactor to an upper portion of the second reactor. A methanol solution containing a final saponified product is drawn out from a lower portion of the second reactor. Consumption of the alkali catalyst can be reduced further by supplying a methanol vapor from at least one of the lower portions of the first reactor and the second reactor, and by saponifying while discharging the methanol vapor, along with methyl acetate as a by-product of the saponification, from the upper portion of the reactor to which the methanol vapor is supplied.
An apparatus for producing EVOH in accordance with the present invention is an apparatus for saponifying EVAc in a methanol solution containing an alkali catalyst, and it includes a first reactor having a copolymer supplying pipe through which a methanol solution of EVAc is introduced and a catalyst supplying pipe through which the alkali catalyst is introduced; a second reactor having a final saponified product discharging pipe through which a final saponified product of EVAc is drawn out; a partially-saponified product delivering pipe that connects the first reactor and the second reactor so as to allow a solution to be delivered and supply the partially-saponified product of EVAc from the first reactor to the second reactor; and a pressure-regulating valve for regulating the pressure in the second reactor.
The method of the present invention can be carried out continuously using the apparatus of the present invention. This apparatus is suitable for industrial production of EVOH having a high saponification degree, using a small amount of alkali catalyst.
For the apparatus of the present invention, the first reactor and the second reactor can be tower reactors, where a copolymer supplying pipe and a catalyst supplying pipe are connected to the upper portion of the first reactor, and a final saponified product discharging pipe is connected to the lower portion of the second reactor, and the first reactor is connected at the lower portion to the upper portion of the second reactor via a partially-saponified product delivering pipe. In this case, a methanol vapor supplying pipe can be connected to the lower portion of at least one of the first and second reactors, and a methyl acetate discharging pipe can be connected to the upper portion of the at least one reactor.